1. Field of the Invention
The present invention relates to a crystallization method for obtaining high-purity products by crystallization from liquid mixtures containing crystallizable components and, in particular, to a crystallization method suitable for large-scale and multi-stage purification of acrylic acid and methacrylic acid.
2. Description of the Prior Art
For example, commercially produced acrylic acid usually contains such impurity components as acetic acid and propionic acid. The concentrations of these impurities are about 0.1% in total. However, due to recent expansion of usage, very high-purity acrylic acids with impurity concentrations in the order of some tens to hundreds of ppm are required, for example, for paper diapers.
In general, impurities are removed by distillation. However, it is very difficult to remove such impurities as acetic acid and propionic acid by distillation since these impurity components have boiling points close to that of acrylic acid. Under such circumstances, it has been proposed to remove these impurities by crystallization.
Two typical crystallization methods are available; one being that seed crystals are put into a liquid mixture containing a crystallizable component so as to nucleate and grow crystals in a suspended state in the liquid, and the other being that crystals are grown on cooled surfaces. In case of large-scale processes, particularly when producing a large quantity of adhesive crystals, such as acrylic acid, the latter method is suitable for it, but the former method is not. The technique employing the latter method for purification of acrylic acid and methacrylic acid is disclosed in, for example, Japanese First (unexamined) Patent Publication No. 7-48311 and Japanese First (unexamined) Patent Publication No. 7-163802.
The former publication discloses a technique, wherein a dynamic crystallization apparatus and a static crystallization apparatus are combined so as to perform multi-stage crystallization. In the dynamic crystallization apparatus, acrylic acid containing impurities (feed liquid mixture) is introduced to flow down on an inner surface of a vertical tube and, by cooling the surface, the crystals grow on the inner surface. On the other hand, in the static crystallization apparatus, the mother liquor obtained through the dynamic crystallization apparatus is introduced into a tank equipped with cooling coils and crystals are formed on the surfaces of the cooling coils.
The latter publication discloses a technique, wherein multi-stage crystallization is performed using a crystallizer having vertical plates and tanks. Specifically, a liquid mixture is introduced to flow down on one surface of the plate and, by cooling the plate from an opposite side, crystals are formed on the one surface of the plate. Further, the mother liquor, the sweated liquid and the melt obtained through crystallization using the liquid mixture in one of the tanks as a feed are sent to different tanks, and obtained melt is further crystallized.
On the other hand, it is possible that polymerizable substances, such as acrylic acid or methacrylic acid, are subjected to polymerization due to, for example, temperature rise or contamination with substances such as iron rust which can initiate polymerization. Further, heat generated by polymerization possibly causes vigorous reaction or explosion. In view of this, a polymerization inhibitor is added in advance in a polymerizable feed liquid before crystallization. The polymerization inhibitor is also added in advance in a final purified liquid (product).
Experiments conducted by the present inventors have revealed that the polymerization inhibitor is not contained in the crystals in the course of crystallization, but concentrated into the mother liquor. Thus, if the crystals containing substantially no polymerization inhibitor are melted, polymerization possibly occurs upon melting crystals, upon transferring the melt or during storing the melt in a tank which constitutes the multi-stage crystallization system.